Factor the following expression: $-6$ $x^2+$ $25$ $x$ $-21$
Explanation: This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-6)}{(-21)} &=& 126 \\ {a} + {b} &=& & & {25} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $126$ and add them together. The factors that add up to ${25}$ will be your ${a}$ and ${b}$ When ${a}$ is ${7}$ and ${b}$ is ${18}$ $ \begin{eqnarray} {ab} &=& ({7})({18}) &=& 126 \\ {a} + {b} &=& {7} + {18} &=& 25 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {-6}x^2 +{7}x +{18}x {-21} $ Group the terms so that there is a common factor in each group: $ ({-6}x^2 +{7}x) + ({18}x {-21}) $ Factor out the common factors: $ x(-6x + 7) - 3(-6x + 7) $ Notice how $(-6x + 7)$ has become a common factor. Factor this out to find the answer. $(-6x + 7)(x - 3)$